Macroscopic Quantum Tunneling and Coherence
Does Quantum Mechanics break down for big (macroscopic)-objects?
that is:
can an object large on the scale of atoms be found in a quantum
superposition?
Most physicists probably think the theory
will still hold up, but
that has important philosophical consequences. In the last decade, Tony Leggett
and collaborators have written a lot about this, and have stimulated a number
of interesting experiments on SQUIDS. These
small superconducting devices
support currents in a loop structure, the
idea being that the current flow
can be in a
superposition
of clockwise and anti-clockwise directions. This
they call "Macroscopic Quantum Coherence
(MQC)", a fascinating phenomenon
not yet convincingly seen in experiments.
A related effect is the tunnelling
of current from a metastable state
in SQUIDS. In contrast to MQC,
this "Macroscopic Quantum Tunnelling
(MCT)" has been
experimentally observed.
See A.J. Leggett in Quantum Tunneling in Condensed Media,
eds. Yu Kagan and A.J. Leggett, (Elsevier Science 1992) for a recent
discussion
of the theory, and M.H.
Devoret in the same volume for a discussion of the
experiments.
Last modified: May 31, 1994
Ard Louis, ardlouis@lassp.cornell.edu